An example of a conventional apparatus for estimating the internal state and the parameters of a battery is disclosed in JP 2012-58089 A (PTL 1). This conventional apparatus for battery parameter estimation detects the charge/discharge current and terminal voltage of a battery, and with these values as inputs, estimates (calculates) the parameters and the internal state quantity and open circuit voltage value of the battery with a Kalman filter using a battery equivalent circuit model. A Foster-type RC ladder circuit, for example, is used as the battery equivalent circuit model.